commit 8dd1646f0b87c49c28b1d7ac023fb0522d1fb942
author Neels Hofmeyr <nhofmeyr@sysmocom.de> Thu Jan 13 20:06:53 2022 +0100
committer Neels Hofmeyr <nhofmeyr@sysmocom.de> Sat Mar 18 03:05:34 2023 +0100
tree d71ab49dc7a311b8c5b5ce87365b612a2d8e64c8
parent 006b0ee50a8454c61815394323dd997f8e91f28d

[codecs filter] send + receive SDP via MNCC

Transmit and receive full SDP information via MNCC, to accurately pass
codecs choices between the call legs.

In msc_vlr_test_call.c test_call_mt(), show that when receiving MNCC,
the codec information in SDP overrules the Bearer Cap codec information
-- we expect to still receive inaccurate Bearer Cap from e.g.
osmo-sip-connector, because we have chosen to add SDP to MNCC instead of
trying to fix the codecs represented in Bearer Cap.

For internal MNCC, the MT call leg now knows which codec the MO has
chosen and assigned.

For external MNCC, osmo-sip-connector receives SDP about our codecs
choices and sends it in SIP messages, and we also receive the full SDP
information from the remote SIP leg.

Update the SDP in codec_filter every time it is received, to always have
the latest SDP information from the remote leg.

 CC              MNCC
 | ---ALERTING--> |     add local side SDP to MNCC msg
 | <--ALERTING--- |     store remote side SDP
 | <--SETUP-RESP- |     store remote side SDP
 | --SETUP-CNF--> |     add local side SDP to MNCC msg
 | -RTP-CREATE--> |     use codec_filter, add local side SDP to MNCC msg
 | <-RTP-CONNECT- |     store remote side SDP

There still is one problem: when initiating MNCC, we do not yet know the
RTP address and port to be used for the CN side, because the CN CRCX
happens later. So far we send 0.0.0.0:0 as RTP endpoint in the SDP,
until the CN CRCX is done. A subsequent patch moves CN CRCX to an
earlier time, adding proper RTP information right from the start.

Related: SYS#5066
Change-Id: Ie0668c0e079ec69da1532b52d00621efe114fc2c